1. Field of the invention
The invention relates generally to optical fibers and, more specifically, to optical fiber waveguides formed of a non-linear optical (NLO) polymer which has been electrically poled to induce a permanent, noncentrosymmetric molecular orientation.
2. Description of the Prior Art
Optical fiber waveguides are now widely used in a variety of applications. For example, optical glass fiber transmission lines defining an optical waveguide are now used to transmit voice or data signals in the form of a modulated light beam. Such optical fibers have been fabricated of a glass core of a relatively high index of refraction, surrounded by a glass cladding having a relatively lower index of refraction, and typically have a diameter of about 125 microns, although they may range up to 1000 microns, for a multi-mode waveguide. These fibers may then be coated with a protective material to protect the glass fiber from abrasion and enhance the structural properties for handling in the field. Materials commonly employed for optical fibers are usually inorganic.
Devices for use in optical communications, such as modulators, switches, multiplexers and demultiplexers have been fabricated by coating inorganic crystals, such as niobates and tantalates, having non-linear optical properties, with conductive layers whereby the index of refraction may be varied in accordance with an applied electric field or control signal. However, low device yields, a high dielectric constant and degradation induced by the laser light source have resulted in consideration of other techniques and materials. Semiconductor devices integrated directly with a laser source have been found to have low efficiency, and the electro-optic coefficients and modulation bandwidth are less than needed for high-speed and high-channel capacity.
Planar electro-optical (E-O) devices in which an applied voltage induces a change in refractive index substantially proportional to the applied voltage are well known in the art. See, for example, U.S. Pat. No. 4,767,169, "Thin Film Waveguide Electro-Optic Modulator". Such devices can be constructed to form waveguides, switches or modulators, for example, and may utilize interference effects, directional coupling or rotation of the plane of optical polarization. Extensive background studies may be found in "Non-linear Optical Properties of Organic and Polymeric Materials", D. J. Williams, Ed., ACS Symposium Series 233, Washington, D.C. (1983), which is hereby incorporated by reference.
While electro-optical devices utilizing bulk-grown inorganic crystals as the propogating medium are well known and widely utilized, they are difficult to grow and process and are limited in bandwidth response. Planar E-O waveguides have also been constructed using certain organic and polymeric materials which exhibit substantial non-linear responses when poled, and provide damage-free thresholds against the operative electric fields and applied laser beams. However, after formation, the delicate polymer films are subject to mishandling and mechanical damage. In addition, the high-intensity electric fields necessary to polarize the medium (e.g., field strengths in excess of 100,000 V/cm) can induce structural damage to the polymer.
U.S. Pat. No. 4,887,884, assigned to the assignee of the present invention, provided a structure in which an optical polymer was encapsulated in an optically transparent hollow fiber to provide physical protection during poling and subsequent handling without impairing the desired optical properties. However, this device had a limited length of 10 to 15 cm and was not suitable for fabrication in extended lengths.
The present invention provides for fabricating a non-linear polymeric material in the form of a continuous optical fiber, thereby forming an optical waveguide of low loss and arbitrary length. Advantageously, the fiber geometry provides the necessary optical and physical compatibilities with existing optical circuit elements to form frequency doublers, parametric amplifiers, phase shifters and modulators, as well as low-loss transmission lines for use in multi-node networks. Remote fiber sensing of electric fields is also contemplated.